A number of different types of Scanning Probe Microscopes are in existence. They operate on different principles, depending on the intended application. Examples include the scanning laser microscope, the scanning thermal microscope, the scanning capacitive microscope, the magnetic force microscope (MFM), the atomic force microscope (AFM), and the scanning tunnel microscope (STM). They are used mainly for surface studies.
For example, the STM is used to study the topography of conductive surfaces, by monitoring changes in the tunnelling current between the surface and a sharp tip. AFMs are usually used to provide 3-D images of non-conductive surfaces, including surface absorbates, via atomic force (van der Waals) attraction and repulsion between the AFM tip and the surface atoms. MFMs are generally used to detect the presence of magnetic materials, via usage of a permanent magnet.
None of the above described microscopes are capable of detecting the presence of small metallic patches present in the surface of a semiconductor or an insulator. As will be described in detail below, the present invention achieves this through a combination of eddy current generation and magnetic detection. In searching the prior art we have not come across any devices that detect metal patches in this manner. The following references were, however, found to be of interest:
Miller et al. (U.S. Pat. No. 4,190,799 Feburay 1980) describe a non-contacting method for measuring the Hall effect in a wafer. An eddy current is induced in the wafer from a pair of concentric co-planar electrodes. A strong external magnetic field is applied perpendicular to the electrodes. The Hall effect then leads to the presence of a circular component of RF current in the wafer (whose center is the inner co-planar electrode). This, in turn, generates an RF magnetic field (superimposed on the DC field) that is detected by a pickup coil.
Look et al. (U.S. Pat. No. 4,857,839 August 1989) measure the Hall effect in a wafer by successive rotation of the positions of all contacts (those used to supply current and those used to detect the Hall voltage).
Watanuki et al. (U.S. Pat. No. 5,266,897 November 1993) describe a tunnelling microscope having a tip made of magnetic material. When scanned over a region containing magnetic material, the tip is attracted to, or repelled from, the surface. The change in its distance from the surface is reflected as a change in the tunnelling current between the tip and the surface.